If you want to keep your existing factory radio, you can get a Direct FM Transmitter. You can get this unit at most Best Buy stores. This is the product link:
http://www.bestbuy.com/site/iSimple+...ract_desc=null
It is fairly easy to install with some basic knowledge. I tapped into my accessory wire that runs in the door jam on the bottom side of my drivers side door to provide power to the device. When I shut off my car and open my door my radio shuts off, at that point it also shuts off the power to my direct fm transmitter device and will not worry you with draining your battery or anything like that.
The kit comes with an auxiliary cable and an iPod connection cable. If you use the iPod connection cable it will also charge the ipod.
If you have any questions about it just message me.